For example the world is too large, or the people and characters are so wild that the costuming would be horrible. Or the CGI just hasn't got to the point where it would do justice to the scenes or actors / characters involved?

For me, it's Red Rising. A fantastic future space drama and war opera with castes of humans ruled by an overarching "Gold" Society of battle-hardened strategically-genius mega-humans. It all takes place within our solar system, but the vast differences in sizes of the different castes in The Society and the augmentations they make to their bodies, the weaponry they use, the ship-to-ship battles, and the "Iron Rains" of tens of millions of soldiers making planet fall just could be put on the big screen (yet, maybe with AI it's possible).

The Golds and Obsidians are all between 6.5 to 8.5 feet tall and largely high-mass/muscle, the reds are all 5'6 or shorter and waifs. Finding the people to accurately represent this book and then being able to find the backdrops for terraformed Mars, Venus, Mercury, Europa, Io, etc. would be next to impossible to do it justice.

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Now, what I envision in the future (20-30 years from now), as AI keeps getting better, is being able to plug an entire book into an AI generator and it spit out a custom movie based on the book details and descriptions which could then be edited / customized by production companies for a final / single product to mass-market and put into theaters.

The Dark Tower series has been attempted, but was a massive failure.
Similarly for 'The Stand'

Honestly, I really loathe most movies made from a book simply because a tv show would be so much better.

Each season a book.

A few notable exceptions.

I wish Project Hail Mary had been a 10 episode TV show.

I think Confederacy of Dunces would be impossible to film and have it translate.

Dante's divine comedy would also be really difficult.

I would be pretty surprised if stormlight archive ever got put on the screen. I'm 20-30 years when AI does all the CGI legwork who knows.

Naked Lunch, I know they tried…no way the movie matches that awful book

Akira.

The Malazan Book of the Fallen.

The Chronicles of Thomas Covenant the Unbeliever

The Dresden Files.

Granted, they are short enough you could easily make each one a 2-2.5 hour movie and pretty much hit all the important stuff. However, no one is starting a 20+ movie series from an IP that doesn't have incredible mass appeal, like Marvel.

I'm still hoping we one-day get an HBO or Netflix series... It could be so freaking epic, if done right.

Claude! said:

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The Malazan Book of the Fallen.

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I tried reading that book series a few years back. I got through 3 books and just moved on to other series and never went back to it. I'm a very patient reader, but I remember there being parts of those books that dragged. Perhaps I need to start over and give it another chance, because I know it's a well thought of series.

Hubert J. Farnsworth said:

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Claude! said:

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The Malazan Book of the Fallen.

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I tried reading that book series a few years back. I got through 3 books and just moved on to other series and never went back to it. I'm a very patient reader, but I remember there being parts of those books that dragged. Perhaps I need to start over and give it another chance, because I know it's a well thought of series.

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Some parts can be a slog and others don't make sense for a few books. It's just way too big a story.

If you want something with a similar kind of vibe, at least in terms of the everyday soldier parts, try Glen Cook's Black Company series. An inspiration for Malazan, but much more spare in the writing.